One of the major methods used at the present time in the semiconductor industry for grasping, holding, moving, and positioning of semiconductor wafers is the use of a mechanical hand of a robot equipped with a vacuum chuck.
From the beginnings of the semiconductor industry to the late 1980s, wafers were handled manually and later by rubber-band conveyors and cassette elevators. The first standards for wafer of 2″, 4″, 6″ diameters and appropriate cassette dimensions allowed to develop simple wafer handling mechanisms and standardize their designs. The early forms of automated handling contributed to improved yields by reducing wafer breakage and particle contamination. A variety of equipment layouts were used, but the general conception remained the same. In other words, the automation systems of that time relied mostly on stepper-motor-driven conveyor belts and cassette elevators to eliminate manual handling.
A central track would shuttle wafers between elevator stations that serviced cassettes and “tee” stations that serviced the process stations. This to some extent helped to reduce breakage, but did not solve the contamination problem. Furthermore, most equipment had manual loading as the standard, with the conveyor and elevators added. These systems were reliable and cheap and served as a good prerogative to automation of wafer handling by the times when 200-mm wafers came into use.
Further progress of the industry accompanied by an increase in the diameter of wafer with 200-mm diameter as a standard for substrates led to drastic changes in principles wafer handling occurred. Driven by ever-decreasing linewidths, tighter cleanliness and throughput requirements, and improvements in robotic technology, the rubber-band conveyor/cassette elevator solution was surpassed by true robotic wafer handling.
The new robotics consisted of polar-coordinate robot arms moving wafers with so-called “vacuum end effectors. In robotic, the end effector is a device or tool connected to the end of a robot arm. For handling semiconductor wafers, an end effector may be made in the form of grippers of the types described, e.g., in U.S. Pat. Nos. 5,108,140, 6,116,848, and 6,256,555. More detailed description of these end effectors or grippers will be considered later.
These robots were an improvement over the earlier technology. Since the robot's movements were controlled by microprocessor-based servo controllers and servomotors, it became possible to greatly improve the throughput, reliability, and error handling of the wafer handling systems. For example, a typical rubber-band conveyor and cassette elevator system could handle only tens of wafers per hour, while a three-axis robot could move hundreds. Reliability for robots was increased at least up to 80,000 hours mean time between failures (MTBF) compared to a few thousand hours for the conveyor systems. In the case of emergency situation the operator must immediately locate a wafer. This was not always possible with a belt-drive conveyor that could not always determine a current position of the wafer, whereas a robot system, which was characterized by a few possible wafer locations, could significantly facilitate a solution of the problem and allowed automated error handling.
Introduction of microprocessor control allowed true unattended equipment operation. Operators could manually load cassettes, and the tool could automatically process full wafer lots. Standards also were improved and introduced into use (see, e.g., SEMI standards). However, these standards helped reduce, but did not eliminate, the confusion involved in the selection and application of robotic wafer handling. For example, there are SEMI standards for cassettes, yet many nonstandard cassettes are used. Another compromise is the need to design semiconductormanufacturing equipment suitable for accepting a large variety of wafer sizes. This adds unnecessary complexity to equipment design.
Furthermore, many equipment manufacturers built their own robots. Each model had to be adaptable to many different wafer sizes and a variety of cassettes.
Recent transfer to 300-mm wafers, evolved new problems associated with much higher cost of a single wafer (up to several thousand dollars as compared with several hundred dollars for 200 mm wafers) and thus required higher accuracy and reliability of the wafer handling equipment. These problems becomes even more aggravated for handling double-sided polished wafers, where both sides of the wafer are used for the production of the chip. In other words, only edges of the wafers could be used for gripping, moving, and positioning of the wafers.
Furthermore, transition to 300 mm wafers made the use of low vacuum unsuitableforholding and handling the wafers. The main reason that in order to protect the wafer from contamination through the mechanical contact with holding parts of the robot arm, both sides (front or back) of the wafer becomes untouchable for handling. Another reason is that vacuum holders are not reliable for handling wafers of heavy weight. Thus, the conventional vacuum end effectors appeared to be unsuitable for handling expensive, heavy, and hard-to-grip wafers of 300 mm diameter.
According to Semi Standards, the allowance for the gripping area of the 300 mm wafer should not exceed 3 mm from the edge of the wafer and preferably to be down to 1.5 mm or even less. To reliably hold the wafer and to protect it from breaking during all handling transportation procedures, it is necessary to use a limited holding force of at least at 3 points circumferentially spaced along the edge of the wafer.
Since the position of each cassette and each wafer within the cassette is unique, the location of each wafer within the three planes of the orthogonal coordinate system relative to the reference plane of robot arm should be measured and used for precise positioning of the robot arm that carries the gripper. Using mechanical measurements or preliminary mapping procedures of location of the wafer in a cassette for precise positioning of the gripper relatively to the grasping points is a time consuming procedure that is difficult to perform in real conditions of the variety of wafer stages at wafer handling robotic lines.
U.S. Pat. No. 5,570,920 issued on Nov. 5, 1996 to J. Crisman et al. describes a robot arm with a multi-fingered hand effector where the fingers are driven from a DC motor via a system of pulleys with control of a grasping force by means of strain gauges attached to the inner surfaces of the fingers. However, such a robot arm is three-dimensional and is not applicable for handling thin flat objects, such as semiconductor wafers, located in a deep narrow slots of a multistack cassette of the type used for storing the wafers.
U.S. Pat. No. 6,167,322 issued on Dec. 26, 2000 to O. Hollbrooks, which describes Intelligent wafer handling system, is typical of the state of the art in two aspects. Hollbrooks system removes wafer from the cassette using a gripper that can slip in between parallel stacked and spaced wafers that has one or more actuating rods and one or more rotating fingers which are rotated by 90 degrees. Translator solenoid acting through an arm applies lateral movement to the finger to grasp the wafer between the finger and the posts. Grasping action is accomplished by using the finger to pressure the wafer against the fixed rods. The level of the pressure is maintained through the control of the electrical current applied to the driving translator. Hollbrook claims that the system can locate the position of the wafer with high degree of accuracy by employing light beams and photo sensors. The intelligent wafer handling system consists of a wafer mapping sensor mounted on the wrist end of the hand. The optics of the sensor is comprised of optical transmitters such as lasers or IR diodes and optical receivers such as CCD's or photo transistors used to receive reflections from the edge of the wafer. To determine the position of the front edge of the wafer, Hollbrook recommended using laser distance measuring unit. A laser head located on a two-axis mount would sweep the column of wafers in the cassette. To avoid the misreading of the wafer position, the sensor should span the small focal point across the edge. Hoolbrook recommended to avoid bending or cracking a wafer by lifting the movable finger, controlled precisely by closely controlling current through the voice coil of actuator.
A disadvantage of the wafer handling system of Hollbrooks consists in that this apparatus does not provide control of gripping speed at different stages of the gripping cycle. Another disadvantage of the Hollbrooks system consists in that this system does not provide decrease in gripping pressure when the gripper approaches the edge of the wafer with acceleration.
U.S. Pat. No. 5,504,345 issued on Apr. 2, 1996 to H. Bartunek et al discloses a dual beam sensor and an edge detection system. Two light sources of solid state lasers are used to detect the edges of the wafers in a cassette. The solution proposed by Bartunek et al. to install the wafer mapping sensor on the wrist end hand or on one side of gripper does not solve the problem of detecting the wafer before gripping or during regular mapping process. The wafer placed on the robot arm in front of the sensor would cover the field of view of the sensor. It is impossible to see the wafer in the next slot of the cassette looking above the front side of the wafer if the position of the sensor is determined by the recommendation of Bartunek's patent. But even if the precise position of the front edge of the wafer is known, there is no guarantee that the position of the backside of the wafer is related to the same plane as the front edge. In the meantime any inclination and tilted position of the wafer in a cassette might lead to wafer breakage during the gripping process.
U.S. Pat. No. 6,256,555 issued to Paul Bacchi, Paul S. Filipski on Jul. 3, 2001 shows gripping end effectors for wafer of more then 6 inches in diameter that include proximal and distal rest pads having pad and backstop portions that support and grip the wafer within the annular exclusion zone. The end effector includes a fiber optic light transmitting sensor for the wafer periphery and bottom surface. A disadvantage of the device of U.S. Pat. No. 6,256,555 consists in that this device does not allow to divide the gripping process into several stages with different controllable speeds. In order to prevent jerks at the moment of contact of the gripper with the wafer edge, the last stage of movement of the gripping fingers should be carried out with a reduced speed. The decrease in speed, however, reduces productivity of the gripper's operation. This problem is solved neither by the device of U.S. Pat. No. 6,256,555 nor by any of the previously described devices.
U.S. Pat. No. 5,108,140 issued on April 1992 to S. Bartholet discloses a palm plate and grippers having tactile or other sensors on their upper surfaces to detect the position of the wafer and provide feedback to the control mechanism. A parallel vice-like grip is generated, but there are no means of detecting the real orientation of the wafer relative to the gripper. To control the gripping force directly at the gripping points of the wafer, it is necessary to measure two dimensional coordinates of the plane of the wafer, the relative coordinates, the front and backside edges, and adjust the gripping points to performed measurements and gripping procedures in real time. More problems related to a limited load that robot arm is able to carry and the amount of wires that can deliver the control and sensing signals to a robot controller.
Thus none of the existing robot-arm end effectors is suitable for grasping and moving semiconductor wafers with high precision and grasping force controlled so as to provide soft touch without loss in productivity of the gripping device.